How do I upgrade my iPhone 6 Plus to iOS 14?

Hi,

iOS 12.5.6 is the latest update for the iPhone 6 Plus.

Unfortunately, iOS 13 and higher are not compatible with this iPhone model.

The latest update for an iPhone 6s Plus would have been iOS 15.7 (iOS 16 and higher not supported).

Apple security updates – Apple Support (UK)

If you mean how to upgrade the iPhone 6 Plus to an iPhone 14:

You may be able to trade in the older model, but it would probably have no value (and will be recycled).

iPhone 14 and iPhone 14 Plus - Apple (UK)

Apple Trade In - Apple (UK)